Alphanumeric displays, such as widely used seven- or eight-segment displays, are typically about 3.5 mm thick and have ten or more leads extending from along the edges of the display for attachment to a printed circuit board (PCB.) Typical displays comprise a substrate with seven or more LEDs attached. A molded lens is then added to diffuse the light from the LEDs and define the segments. The lens typically makes up the majority of the thickness of the display.
Typical alphanumeric displays are designed for use with PCBs. However, they are also used in the membrane switch industry, where they must be attached to circuits made by printing conductive inks onto polyester.
A membrane switch is an electrical switch for opening and closing a circuit. It differs from other switches, which are usually made of copper and plastic parts, insofar as a membrane switch may be printed on a thin sheet(s) of material such as a thermoplastic resin of the polyester family, e.g., polyethylene terephthalate (PET). The ink used for silk screening printing is typically copper/silver/graphite filled and therefore conductive. A membrane switch typically has two to four layers that may include: (1) a first PET layer with a circuit printed thereon; (2) a second layer that acts as a spacer, which can be made of PET; (3) a third PET layer having printed thereon a circuit closer; and (4) a top panel, such as a layer of polyvinyl chloride (PVC), with a button pattern, picture, or some other graphic printed thereon. Membrane switches are typically used in electrical appliances in which the rebouncing feeling may or may not be important. Classic applications for membrane switches includes microwave oven control panels, air conditioner control panels, TV remote controls, etc. The tactile feedback of buttons may be provided by embossing the third PET layer or by embedding metal snap domes.
A typical display creates two difficulties for membrane switch manufacturers. The first is that it is too thick. Membrane switches are by their nature designed to be thin, typically less than 1 millimeter in thickness. Because of this, a display is not generally embedded into the membrane switch but mounted on a PCB that is itself mounted behind the membrane switch. The membrane contains only a cutout or a window to accommodate the display. In instances where an attempt is made to embed the display in the membrane switch, it is typically done by creating a flap cut out from the membrane on three sides, which allows the front surface of the display to remain flush with the front surface of the membrane. This creates several problems, including a potential reliability problem with the printed circuit due to the movement of the flap during processing, which could potentially cause cracks in the electrically conductive traces.
A second difficulty created by the design of the existing displays is that the leads are typically arrayed at the top and bottom edges of the display. While that lead arrangement is easily accommodated in the mainstream PCB industry with traditional soldering techniques, the terminal portions of the leads, or “pads,” are typically spaced too close together to reliably dispense adhesive dots for attachment to the conductive ink printed circuits used in the membrane switch industry. While this difficulty can sometimes be solved by screen printing the adhesive, using z axis conductive adhesives, or using proprietary adhesive formulations, all of those techniques create other problems and expenses.